1. Field of the Invention
The invention relates to extraction of alumina from aluminous material of high-silica content. More particularly, it relates to improving the yield of alumina in a caustic-lime digestion of anorthosite and like alumina-bearing siliceous materials.
2. Description of the Prior Art
Caustic-lime digestion of aluminous material of high-silica content such as anorthosite and other ores, shale, clay, Bayer red mud, and the like, is disclosed by Ponomarev and Sazhin on pages 45-51 of a 1957 article entitled "The Hydro Chemical Alkali Method of Processing Nepheline Rocks" in TEVETNYE METALLY; on pages 93-100 of their 1958 article entitled "Leaching of Alumina from Nephelines by Alkaline Solutions in the Presence of Lime" in report of Higher Educational Institutions, Nonferrous Metallurgy; and in their Russian Pat. No. 108,917; as well as I. N. Maslenitiski et al in Autoclave Processes in Non-Ferrous Metallurgy (Chapter I), Izd. Metalurgiya Moscow, 1969.
The process described in these publications involves a high pressure, high temperature digest of nepheline, an alumina-bearing siliceous ore, in caustic using lime to form a digest slurry containing dissolved sodium aluminate and an insoluble sodium-calcium silicate-residue.
It is stated by Ponomarev et al in their 1958 publication that the autoclave containing the digest is cooled to 80.degree.-100.degree. C and the precipitate quickly separated. However, it has been found that, upon reduction of the digest slurry to atmospheric pressure and thereby lowering of the temperatures to atmospheric boiling point (to provide practical temperature and pressure conditions for separation of the dissolved alumina from the residue) a back reaction occurs wherein the alumina in solution reacts with the residue to form a compound having an x-ray diffraction pattern very similar to that of the original ore -- at least when anorthosite is used. These reactions can be expressed (in the following unbalanced equation) as: ##STR1##
Furthermore, it has been discovered that this back reaction surprisingly is favored with reduction of temperature to a maximum reaction rate which occurs at about 100.degree. C at which point as much as 50% of the dissolved alumina may back react with the residue if the residue and solution remain in contact with one another for an hour.
Still further, it has been found that the rate of the back reaction is dependent upon the particle size of the material to be digested as well as the concentration of the digest slurry.
There seems to be little, if any, appreciation of the problem of back reaction in the foregoing Russian publications. While Ponomarev et al 1957 appears to suggest that the slurry should be diluted or washed with water, this may have been merely for recovery of soluble values from the residue. Dilution can create other problems with regard to subsequent evaporation of the added water. The Maslenitiski et al article on page 42 of the translation, even suggests that size of the material to be digested has little effect on the yield of alumina because of the high velocity of the nepheline decomposition.
An alumina extraction or separation method has now been developed which prevents much of such loss of alumina by back reactions.